Wikipedia Preview already includes a `wprov` URL query parameter when it [requests page summaries](https://github.com/wikimedia/wikipedia-preview/blob/main/src/api.js#L36), so that we can count those requests as a proxy for the number of viewed previews. However, currently, we cannot distinguish previews shown on mobile devices from those shown on computers.
To address this, these requests should also be tagged when Wikipedia Preview considers the device a mobile device (based on having touch capabilities; this is different from MediaWiki's definition, but should produce close enough results). In this case, we should add an [X-Analytics HTTP header](https://wikitech.wikimedia.org/wiki/X-Analytics) with the value `wikipedia_preview_is_touch=1`. It isn't possible to use a query parameter because that will cause cache fragmentation; the `wprov` parameter is an exception because there is special cache logic that immediately moves the value into the X-Analytics header.